How To Wire a Thermostat

How do I know which wires to connect to the Thermostat Terminals? Typical Thermostat Wiring for HVAC Furnace Heating and Air Conditioning, Thermostat Wire Connections.

Thermostat Wire ConnectionsElectrical Question #1 I am wiring a thermostat, how do I know which wires to connect to the terminals?

I am purchasing a home which I noticed the thermostat is missing and the wiring is there but no thermostat. I have replaced the one in my current home and used the old thermostat wiring set up to do the new one and it works fine. The thermostat which I am wanting to install is a Honeywell programmable. My question is how do I know which wires go to what terminals since the instructions say do not worry about the colors, use the old one to set up the new one, but there is no old thermostat there.

This electrical question came from: Sid, a Homeowner from St. Lucie, Florida.

Additional Comments: Your website has great articles for the homeowner and the novice.

Electrical Question #2

I’m attempting to install the Honeywell Lyric T5 thermostat but I’m not sure what setting to use. I live in an apartment building that has geothermal heating and cooling and a heat pump in my unit. However, there is no O or B wire that a typical heat pump wiring should have. The wires I have are R, C, G, Y1 and W1 only. When I went to configure the thermostat as Geothermal Heat Pump, it asked for O/B Changeover on Heat or Cool, which neither applies to me. When I chose O/B on Heat, the cooling and heating works but the heat did not shut off even once the ambient temperature has reached the set temperature. I’m confused as to what heating/cooling system I actually have.

This electrical wiring question came from Sanny, a Homeowner in Vancouver, Canada.

Electrical Question #3:

I had a furnace question. I previously switched to a smart thermostat but I’m looking to switch back to the “non-smart” thermostat and I forgot the wiring placement. The system is 1-2 years old and only an AC and heater are connected to it. There are two bundles inside the furnace panel, one has two wires and the other has four. The two wire bundle are white and black The four wire bundle are white, black, red, and green. The furnace board has 7 connectors. They are R, Y/Y2, W, Com 24V, G, DHUM, and Y1. Can you please tell me which wires connect to where? Also, the furnace is a Carrier 59SP5A.

This electrical wiring question came from Robert in Dallas, Texas.

Dave’s Reply:
Thanks for your electrical questions.

Yes Sid, that is a bit odd. The wire colors and number of wires are typically the same as shown below in the thermostat wiring table.

Thermostat Wiring Made Easy

Application: Thermostat for Controlling HVAC Heating Ventilation and Air Conditioning.
Skill Level: Beginner to Intermediate – Best performed by a Licensed Electrician or HVAC Technician.
Tools Required: Basic Electrical Hand Tools including a small screw driver and small wire strippers.
Estimated Time: Depends on personal level experience and ability to work with tools and the small thermostat wires.
Precaution: Identify the circuit for the furnace or heat pump, turn it OFF and Tag it with a Note before working with the thermostat wiring.
Troubleshooting and Testing: Most thermostats may be tested using a Volt/Ohm Meter. The thermostat battery should be tested and replaced as needed.
Notice: Make sure to get the exact replacement thermostat.

Thermostat Wiring Table and Wiring Diagram

Thermostat Wiring Question: Where do the red and white wires attach on a new thermostat?

I have a thermostat now that has only red and white wires and I am wondering where I put them on my new thermostat?

This electrical wiring question came from Sylvester, a Homeowner in Cumming, Georgia.

* Found in some Carrier brand furnaces
**The brown is often the first wire used for the 24 volt common leg however white, black and blue are most frequently used.

Typical Thermostat Wiring for Furnace Heating and Air Conditioning

Most Common Thermostat Applications:

Air Conditioning Thermostat

Furnace Heating Thermostat

Terminal
Label

Wire Color

Function /Description

R

Red

24 Volts

B

Blue

Cooling

G

Green
Grey *

Fan Blower

W

White

Heating

NOTE: Although many home heating and air conditioning systems have a standard type thermostat cable with color coded wires, the wires should be identified to make sure of the exact purpose for each wire.

I have a Nest thermostat connected to control my heat pump. Yellow connected to Y1, Green to G, Orange to OB, Brown to Aux, Blue to C and Red to Rh at the Nest. My problem is that it never stops cooling. Fan is set to always on, and the Nest setting to default except for O/B which is set to O.

Hi Raynald,
Double check the wiring to the Nest compared to the configurations described in the owners manual. Make sure the Functions of Cooling and Heating are not reversed, and check your temperature settings.

The wiring is as described and double checked. The Cooling and Heating are not reversed because all the tests performed with Nest software are OK (heat when supposed to heat, cool, …). Temperature settings are normal (cooling only mode set at 22 C). The problem is that after a while, the temperature inside goes very cold but the thermostat is not sending any cooling command.

Raynald,
If the heat pump is stuck in the cooling mode all the time, then the control wiring to the heat pump will need to be verified.
Make sure the heat pump does not have an other control features that are overriding the Nest thermostat.
Just curious:
What type of thermostat was installed before the Nest, and was the heat pump working OK before?
How old is the home, and how old is the heat pump? Was this unit always working OK?

Dave,
It is not stuck in the cooling mode all the time. I can make it heating by activating the heat/cool mode and raising the target temperature over the ambient. But when it has to cool, it is like it keep cooling even without thermostat command. Or maybe it stops for a while and then start over by itself. All we know is after the night in “eco” mode, it’s 10 C inside in the morning. I am not aware of any other feature that could override the thermostat.

I don’t have the info about the the old thermostat right now, but it was kind of standard programmable thermostat. It was working well with the previous thermostat and has been working perfectly during last winter (Dec. to June) with the Nest up to the first heat wave (few days around 30 C outside). The office building is over 20 years old and the heat pump is a Carrier 38YKC.

Raynald,
I am wondering if the Nest stat has to “learn” the environment and the HVAC equipment, and create a history in order to perform most efficiently. That being said, the control should not allow the environment to go outside of the parameters of your comfort levels as programmed. Something is not right.
Are your settings in Celsius or Fahrenheit?

Everything is in Celsius. So 10 C is 50 F… pretty cold for an office environment. The thermostat has finished to learn, it has been in place and working fine for months before it becomes warmer outside and needed cooling; then it started to behave wrong, cooling way under the limits.

I’m installing a Honeywell rth9580 thermostat. The old original has a red wire connected to RH with a crossover from RH to RC the green wire to g. White wire to w and the blue goes to the y. Where the blue connects to the the screw there is a yellow wire that is connected to the old thermostat it self. But the is no yellow wire that comes out of the wall. The compressor came on before but will not come on now but the fan does.

Hi William,
The wire used on the original thermostat for first stage cooling or second stage cooling must be identified for your system. Focus on the yellow wire of the old thermostat and the RC connection.
I hope this helps,
Dave

I have an Old Hunter Set and Save II Thermostat that has R to R, W to W, G to G, and Blue wire to Y. Four wires coming from central AC/Heat unit. I am trying to use a Honeywell THX9000 stat which doing the same connections there with the R and RC Jumper in place, does not work. As once I pop on the Thermostat face/ screen, I get nothing at all but a blank screen. I have a CR2032 battery in this which has just over 3 volts, checked it. Interesting that I do not see anywhere what Honeywell says to use for a battery either in both manuals provided. It is suppose to come on the screen with a white screen with Honeywell name first, then go to next screen. It stays blank. The circuit breaker is Okay and was On. Double checked connections, okay there also. Leaving it with this connection and circuit breaker off and back on still didn’t change a thing. CB did not shut off at all. What is next to do ?

Hi Dan,
Make sure of the jumper connections as they may be interfering with the new thermostat. Transposing the wire connections from the old thermostat to the new thermostat should be correct. The thermostat batteries typically have a long life, however they can be tested with a volt meter on the DC setting.
I hope this helps,
Dave

I hooked up my new Honeywell thermostat according to the instructions that were provided. The new thermostat seems to work backwards. When I flip the A/C on the switch the actual heater turns on. When I flip the heat switch on, the a/c and condenser turns on. Is there a wire I need to switch or something?

Hi Brandon,
If the A/C unit and the Heater functions are reversed then the connections of two wires need to be reversed. Therefore, on the instruction sheet provided with your Honeywell thermostat, locate the two wires that are connected for the A/C, and the Heater. Disconnect the wires and reverse the connections to correct the A/C and Furnace functions.
Thank you for sharing your Honeywell Thermostat question,
Dave

I am attempting to replace my older thermostat with a Programmable WIFI Thermostat. Instructions state to connect the BLUE WIRE to the C terminal on the new unit and to the C terminal at the furnace. My Y at the thermostat is connected to the C terminal at my furnace. How should I proceed? Is it safe to place the BLUE WIRE over or under the existing Y wire currently there without causing harm? I don’t want to damage anything. Thank you for your assistance.
Nick

Hi Nick,
The instructions for the Programmable WIFI Thermostat refer to a typical wiring arrangement and wire colors, however this is not the case at some installations. Therefore, when we are faced with alternative wiring we must positively the existing wiring configuration and transpose the wiring over to the wiring configuration of the new thermostat. The task is to test the integrity of the wires and their colors.
To simplify things consider that if the original thermostat was working correctly, then we would be safe to adapt the wire colors and their connections over to the new thermostat.
Thank you for sharing your thermostat question with us,
Dave

Dave, Thanks very much for your reply. At my existing thermostat, Rh/RC terminates at R on the furnace bock, W terminates at W and G terminates at G. It’s Y that terminates at C at the furnace that confuses me. I can certainly connect the free floating BLUE wire to the new thermostat. My final thought/question would be, at the furnace terminal block, where the Y(yellow) is connected to C, which is where the Blue should go for power, is it safe to connect the BLUE wire and leave the existing Y(yellow) wire on the C? So both the Blue and Yellow wire would be connected to the C terminal at the Furnace.

I will never be able to convince my wife to pay a professional $150 to install a $150 thermostat – It may be best to simply return the unit.

Follow up – Connected the C (blue wire) to the C terminal at the furnace, along with the Y (yellow wire), which also was connected to the C terminal and everything works fine. Thank you again for your guidance.
Nick

I am trying to install a new thermostat, but I have no green wire only Red, white, yellow and blue. The old Honeywell thermostat had the Blue in the G terminal, but that does not even work now for the new or old thermostat.

Hi Genni,
When wiring a replacement thermostat the colors of the wires and the connections will need to be transferred from the existing thermostat to the new thermostat. Because not all thermostats are alike, it is important to make sure the replacement thermostat is identical or fully compatible with your heating and cooling system, and serves as a replacement for the original thermostat.
Thank you for sharing your thermostat question with us,
Dave

I purchased a Honeywell thermostat a couple years ago, and it took some splicing and rewiring at the furnace to get it working. The new place I moved to has an orange wire as the common, green wire for the fan, and red and white wires. I have no clue which wire powers my air conditioner. The old thermostat did power it but was not labeled. Any way or possible ideas to figure this out?

Hi Devin,
The thermostat wire colors should be identified at the air conditioner unit which will help you make the proper connections at the new thermostat.
Thank you for sharing your thermostat wiring question,
Dave

I purchased a lyric thermostat and followed the instructions to install. Now it will not power up. I traced the wires back and they seem to go from red and white then in basement red goes to black and then back to red. Any suggestions?

I am waiting for Honeywell to open but am lost. Sure, 15 minutes to install!

Hi Joseph,
I believe the instructions for the Honeywell thermostat are accurate under ideal installations where the wiring has not been altered or spliced. You have discovered that your thermostat wiring has been spliced and the color sequence changed, therefore it may be helpful to chart out the wiring to understand the actual purpose of each individual wire. Once this information is identified then the correct connections to the new thermostat will be possible.
I hope this helps,
Dave

I recently replaced my old mercury thermostat with a new programmable Honeywell thermostat. However, the wiring is a bit different. There are 4 wires coming out of the wall – Green, White, Orange and Blue.

For the old thermostat, the blue wire went to “RH” and there was a red jumper from RH to RC. So I did the same with the new thermostat.

On the old thermostat, the orange wire went to the “Y” terminal. And then a separate yellow wire went from the Y to the back of the wall plate. But that yellow wire was removable.

I connected the orange wire to the “Y” on the new thermostat. Is this correct? Or do I need to take that separate yellow wire from the old thermostat plate and have it connect somewhere?

Hi Alli,
From what you are describing you may not need a jumper wire for the new thermostat, however this will highly depend on the type of heating and cooling system you have, and the specific components that are in the system. Compare the wiring to the tables provided above, and most of all the instructions and wiring diagrams that came with the new thermostat.
I hope this helps,
Dave

Hi.
I replaced a simple two wire thermostat four days ago and the furnace seems to be running normally. However, the fan/blower is vibrating all the time now. Could this have anything to do with the new thermostat? Why would it start to make this noise right after replacing the thermostat?

Hi Deb,
Your right, replacing the thermostat will only take care of turning the furnace on and off.
The furnace blower vibration may be due to filters that are in need of replacement. Air flow restriction due to obstructed furnace filters can put a strain on the blower unit and will affect the efficiency of heating or cooling the home. There is typically a furnace filter located behind the return vent grill or register which is normally located in the hallway in the ceiling or on the wall. There may also be a filter inside the furnace blower compartment, it just depends on which type of furnace you have.
Also, the blower unit may be in need of lubrication. Some furnace blowers have lubrication points on each side of the blower shaft.
It would be best to have the furnace unit checked out along with the ventilation duct work as well to make sure the ducting is not restricted and sealed properly to ensure normal air flow.

I bought a new thermostat because my old one’s display was no longer working. I labeled all wires but when I went to hook up the new thermostat a few of the wires didn’t have terminals to go to, or at least the labels didn’t match any of the terminals (for instance no C). I have 8 wires total coming out of the wall going to thermostat. 5 wires are coming out of one cable, and 3 wires out of another. The 5 wire cable has red, green, white, blue, and yellow. The three wire cable has red, white and green. I have looked everywhere on line to try and figure out what to do, but all of them only used 5 wires. Do you have any idea what I need to do?

Hi Alex,
I have run into similar thermostat installations, and here is what typically helps me:
First of all, you will need your old thermostat and the label information you created before removing the original wiring.
On a sheet of paper, make a 2 column table.
In one column of the table write down the original wiring connections, noting the terminal label and the wire color for each cable.
On the other column of the table record any known labels that are identified on the new thermostat which logically match the original thermostat.
As you do this procedure notice the labels and designations on the thermostat and you may begin to notice the logic of what the wires are used for compared to the specific HVAC system that you have.
It will help to understand that not all HVAC systems are alike, or have the same components. However the function of heating and cooling is still the same, but may involve different system components.
As for any left over wires, do you remember any spare wires when you first started? Or were any wires spliced together and not attached to the original thermostat?
It is likely that the 5 wire cable goes directly to the main furnace unit, and the 3 wire cable goes to another component of the system, such as a air conditioning compressor, which obviously would be part of the cooling mode. However, the only way to positively identify all of the wires of each cable would be to find the other end and note each wire connection, and this information could actually be used for a 3rd column of information in your table.

I have an issue with a thermostat wiring. I tried to install a wifi thermostat and when I took the old one off the wall it had the typical Red, yellow, white and green wires, but it did not have a “C” for the wifi thermostat to function. I noticed an unused blue wire and when I opened the furnace control panel I noticed that the blue wire was not connected to anything on that end either. So I connected it to the C in the furnace panel and then to the C in the thermostat. It worked great….except that after a little while I noticed the house was not cooling off. The fan was working, but the condenser unit outside was not turned on. I looked at the breaker box and it was tripped for the condenser unit. I flipped it back on and it worked….for a little bit and then tripped the breaker again in a couple of hours. It was getting too hot to function, so I just turned off all the power, put the old thermostat back on the wall wired exactly as before, and disconnected the blue wire in the furnace control center again. I put everything back exactly how I found it until I could think about it so more. I turned the power back on and everything worked great! The house was back down to a really cool setting again. After a day though, the breaker for the condenser unit tripped again. Now I really dont know what to do. It keeps tripping and making he house unbearably hot and then we have to go out and flick the breaker just to get it working again. Did I screw something up? I double checked that I tightened the C connection tight again in the furnace box.

Hi Mark,AC Unit Problem and Thermostat Wiring
I doubt that the problem with the AC condenser unit is due to replacing the thermostat. The condenser unit should be tested by a HVAC technician, and the freon level should be checked as well. Make sure the condenser unit coils are clean to provide unrestricted air flow through the coils.
I hope this helps,
Dave

I am replacing an older Honeywell thermostat with new Honeywell stat. The old one was wired red to r, white to w1, yellow to y1, green to g, black to c. On the new stat there is no terminal labeled “c” and it has a jumper from R to RC. Do I just eliminate using the black wire on the new stat and keep everything else the same? Thanks.

Hi Joe,
From what you have described, your new Honeywell thermostat does not require the C terminal connection, therefore cap off the black wire with a small wire nut or wrap it with electrical tape and keep it as spare. Test the unit for proper operation. If you find that the system is not functioning properly then you may have to verify the wiring. Hang on to the old thermostat just in case you need to compare the old connections and functions with the new thermostat. It’s always best to fully understand the function of each wire, which can then be verified with the functions associated with each wire terminal of the thermostat.

I replaced my old thermostat with a Honeywell RTH25 10B. I had problems with it last winter as the heat would not turn on. After trouble shooting the problem I called my local plumbing co, and after testing all the components the gent checked the thermostat and found that it was miss-wired, all to the tune of $168.00. Now that Summer is here my A/C is not working. I again did some trouble shooting and found that when I checked voltage across the contacts coil it read 0 VAC. I again checked wiring at the t/stat and it looks correct, especially when I compare it to the diagrams on this web page.

Hi Rob,
I would first check to make sure the thermostat is switched over to Cooling mode, and that it is not in the Heating mode. Also make sure that the temperature settings are correct. I know that sounds simple, but these basic settings are often overlooked.

Is your A/C unit being controlled by the Smart Meter program through your local utility company? If so the Smart Meter can keep A/C units from running during high peak electrical demand periods during hot days. Contact your local electrical utility provider for more information about Smart Meter programs in your area.

If you find that the thermostat is set correctly then consider contacting the company whom you previously paid to rewire the thermostat and ask them if they guarantee their work and allow them the opportunity to correct the thermostat problem.

As described in this article for wiring a thermostat, each thermostat control wire of your HVAC system will need to be identified so the thermostat may be wired correctly.

I just installed a programable honeywell thermostat to both my upstairs and downstairs units. However my upstairs unit will turn on but it works backwards when the heat is turned on it will cool and when it is set to cool heat comes out. I tried switching the wires on the thermostat but it didnt change. Any ideas what else the problem could be?

Hi Chris,
Double check the thermostat wiring. Keep in mind that for the heating and cooling cycles there are actually three wires involved, the Power Supply, the Heating Cycle, and the Cooling Cycle. If any one of these wires are not connected properly the cycles will either not work, or the cycles will be incorrect.
Also make sure that the Cooling and Heating Modes are set up correctly at the thermostat. Programmable thermostats are great, but they can be challenging to set up, and the default programming can be tricky to understand as well.
I hope this helps,
Dave

I’m trying to connect my parents thermostat but the person prior to me installed it wrong and didn’t label it. I have blue, green, orange, brown, but no red, a lot of white blue, white orange, white brown, and white green wires. I need help to connect a Honeywell programmable thermostat.

Hi Frank,
When the colors of the thermostat wires are not standard then a conversion chart or diagram will need to be made to identify the wire colors that you have and how they will be connected. It is possible that the original thermostat cable has been spliced to the wires that you are describing. The charts on this page will help you understand the typical functions for each wire, and by using a voltage tester the power source may be identified, and the additional functions. It would be very helpful to examine the thermostat wiring diagram and the wiring as it is connected to the control board at the furnace which will provide a clear understanding of the function for each of the thermostat wires.
I hope this helps,
Dave

I put in a new digital thermostat to replace an older mercury switch type thermostat. The furnace runs. A service man said the red and white wires were reversed. Will the furnace still turn on and get hot with this reversal?

Hi Donald,
The red wire typically provides the 24 volt power source for a heating and cooling thermostat. If the white and red wire were reversed this could prevent all of the thermostat functions from working properly, especially when in the cooling mode. From what you have described, the serviceman made the right corrections to enable the thermostat to work properly.

I’m replacing an old thermostat to a new Honeywell rth221. The old thermostat had 4 wires: white, red, green, and black. The white wire connected to w, red to r, green to g. Where do I connect the black wire? P.s. it’s an HVAC unit.

If the HVAC functions work for the fan and the heat, then the only function left would be the cooling, which is commonly the blue wire of the thermostat cable. Keep in mind that there may be thermostat cables with different wire colors, therefore it will be best to check the connections of the cable at the HVAC unit to see how the thermostat wire colors are assigned for the functions of the HVAV unit. The wiring diagram on the HVAC unit will be helpful as well.
I hope this helps,
Dave

On my furnace, a white wire was attached to the C terminal and then pig tailed to a red wire. A green wire goes to G, a red wire goes to R, and a white wire goes to W. (I do not have air conditioning.) A white wire then is pig tailed to the green, red, and white wires. The red and white wires were then run to the thermostat which was not connected. When I moved in, I re-connected the thermostat, hooking the white wire to the W terminal and the red wire to the R terminal. I then unhooked the wires that were hooked up to the furnace but recorded where they had been connected. I connected the red wire to the R terminal and the white wire to the W terminal. The furnace runs great as does the blower BUT the blower will not shut off and runs all the time. When I set the temperature on the thermostat the heat goes on and off but the blower keeps running.

My question is: I do not want the blower to run when it is not calling for heat. Should I hook up the wires the way they had them connected? Also if I do, should I connect the pig tailed white wire to the W terminal and pigtailed red to the R terminal or should I reverse them connecting R to the white wire and W to red wire? Please advise me! Thanks in advance!!!

Hi Roger,
From what you have described about this low voltage thermostat wiring the blower motor has been wired direct and that’s why it is running all the time, which could be because there is a direct connection between R and G.

The basic operation that takes place when a furnace is calling for Heat is where R (the positive wire of the low voltage power supply) makes contact with W (the first stage heat).

Normally the blower is controlled by a switch at the thermostat with the positions for FAN – OFF – AUTO. When this switch is in the AUTO position, or with thermostats that do not have this switch option, the fan blower motor will operate automatically by heat sensors in the furnace when the heater is working.

IMPORTANT:
It is important that the wiring at the thermostat will only work right if the low voltage thermostat wiring at the furnace is connected correctly.

I re-hooked the thermostat back the way the wires were on the furnace but the blower still runs all the time. On the thermostat there was no manual or auto fan switch. So I got a programmable thermostat. I plan to hook red to rh and a jumper to rc, g to green, w to white, y to yellow, on the thermostat. On the furnace should I hook them up the same way? Should I hook a wire to the c post? If you recommend I hook to the c post which color should I hook to the c? Do you think this will solve my problem?
Thanks.

Hi Roger,
Keep in mind that the wire connections of the typical thermostat comply with a standard for most systems, but there are deviations, so this is where verification is important. The verification has to be made at the furnace unit itself, starting with the schematic, specifically the low voltage thermostat portion of the schematic.
Roger, if I were dealing with this situation, I would go to the furnace unit, check the schematic diagrams as described in the preceding, and then work directly with the low voltage thermostat wires at the unit and manually make the connections and watch the unit operate. Once the operation has been verified and the furnace unit is operation normally then I would connect the thermostat cable and make the appropriate connections to the thermostat.
I hope this helps,
Dave

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